Why did a lot of big IPs never receive much doujin attention?

>>8653014 (OP)
Copyright. In Japan, h-doujins are technically copyright infringement. A lot of companies will turn a blind eye because it can be good for business depending on the subject of parody. However, there's also many companies explictly hostile towards h content of their IP. Pokemon being a primary example where a prolific artist got arrested after she penned copious amount of pokemon hentai.
Worth noting that this is largely only a problem for Japan, as the US and other countries have laws that protect such parody.

>>8655381
Pokémon isn't a very good example, there is plenty of Pokémon doujinshi. Tamagoro alone has plenty of Pokémon hentai. That said, the companies can have the last word on doujinshi. I remember when SSSS.Gridman came out, Studio Tsuburaya actually made the request that erotic doujinshi not get made--at least, not at first. Folks complied, because a lot of fan-based doujinshi exists at the whims of license holders.

>>8655381
interesting

>>8655381
TIL